Fix using monomorphic store instruction for polymorphic stores.

src/hydrogen.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 6390 matching lines @@
     }
   }
 
   if (count != types->length()) return false;
 
   // Everything matched; can use monomorphic store.
   BuildCheckNonSmi(object);
   AddInstruction(HCheckMaps::New(object, types, zone()));
   HInstruction* store;
   CHECK_ALIVE_OR_RETURN(
-      store = BuildStoreNamedField(object, name, value, types->at(0), &lookup),
+      store = BuildStoreNamedField(
+          object, name, value, types->at(count - 1), &lookup),
       true);
   Push(value);
   store->set_position(expr->position());
   AddInstruction(store);
   AddSimulate(expr->AssignmentId());
   ast_context()->ReturnValue(Pop());
   return true;
 }
 
 
@@ skipping 5230 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify(false);  // No full verify.
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal